This invention relates to the subject matter of U.S. Patent Nos. 4,679,315 and 4,696,102, both of which have been patented by the inventor of the instant application and are assigned to the same assignee as the instant application.
As stated in those patents, tube and sheet heat exchangers, for example those used in a nuclear steam generator encounter problems in the form of tubes which become defective or damaged. In the past, these tubes were removed from operation by welding plugs into each end during a maintenance shutdown, and thereafter continuing the later operation of the steam generator with reduced capacity with the remaining tubes in operation. This form of maintenance is satisfactory until so many of the tubes are plugged as to make it economically unfeasible to continue operation at the reduced capacity. Thus, instead of doing the above, the tubes are presently plugged by plugs that can be removed at a later time. These plugs are mechanically expanded into the tubes without any welding being done. When too many tubes have been removed from operation, the unit is shut down, the plugs are removed and the tubes are inspected and tested to see which of the defective tubes can be effectively repaired, for example by sleeving. If enough tubes can be repaired, the useful life of the steam generator can be greatly extended. The plugs may be loosened by means of a tool which spirals an electrical arc along the inside surface of the plug wall. As the arc melts portion of its inner surface, the forces of solidification cause the wall of the plug to contract or shrink back, close to the original size that it was before it was expanded into the defective tube. A gripping mechanism is then used for pulling the plug from the tube end. Care must be taken during this action so as not to cause damage to the tubes. Another technique for removal of a plug of the type having internal threads involves merely attaching a pulling tool to the interior portion of the plug and pulling the plug from the tube. This frequently has created tube damage.
The internally threaded type of plug, typically, is Inconel of a first hardness and includes an interior cavity having a threaded portion and a displaceable cone of a second and greater hardness for movement downwardly along an internal tapered surface of the plug cavity to create a component of force which radially expands the softer plug and holds it in place in sealing engagement with an inner surface of a defective tube. Inconel is nominally 13% (by weight) chromium, 79.5% nickel and 6.5% iron. The displaceable cone is typically made of hardened "17-4 P.H." stainless steel. This material has a very low carbon content (0.07% max by weight) and is initially soft. Its other constituents are, typically, chromium (15.5-17.5%), nickel (3-5%), manganese (1% max.) and copper (3-5%). The alloy's ferritic structure permits hardening to 40 to 45 Rockwell "C" by a simple aging treatment at 900.degree. F. to 1,150.degree. F. The plug to be removed may be an Inconel " 600 or "601" material having a typical yield strength of approximately 35,000 PSI. It is a softer than the "17-4 P.H.", generally being below 32 Rockwell "C".